Peak current control is a regulation technique applied to switching regulators or converter circuits to periodically respond to a peak current value in the power switch to terminate the current conduction interval in that switch. Current level in a power switch of a switching regulator increases linearly after a clock pulse initiates the switch's conducting interval. This increasing current value is continuously compared with a reference current level and the switch is turned off when this reference level is attained. This reference level value may be variable in response to an error voltage dependent on the regulators output voltage to achieve voltage regulation or it may be a current related value to achieve current regulation. A detailed discussion of peak current control entitled "Simple Switching Control Method Changes Power Converter Into A Current Source" by C. Deisch and published in the record of the Power Electronics Specialists Conference, 1978, discloses the peak current control method in detail.
Because of storage charge in the flyback diodes and the current inertia effect of the filter inductor, a current spike is generated at the leading edge of the power switch's current waveform when the clock turns the power switch on. This current spike is filtered in a low-pass filter before the current level is coupled to the comparator, in the peak current control, comparing its level with the reference level. This filtering is necessary to prevent the peak current control from responding to the amplitude of the spike and immediately turning the power switch off. This filtering arrangement, however, introduces a sensing error in that the current perceived by the comparator differs from the total current flowing through the power switch. During normal operation, this error is very slight, but during short-circuit conditions when the current pulse is very short in duration, this error becomes a significant portion of the total current flowing through the power switch. The current perceived at the comparator is much less than the actual current flowing through the power switch. Hence, during short-circuit current limited conditions, a current tailout effect occurs wherein the actual current output is much greater than its theoretically regulated value.
This current tailout effect or excess current represents a large surge in current that causes large stresses in the power switch and other components of the converter and, in effect, nullifies to a great extent the limiting effect of the current limit control.
A subsidiary problem caused by the current sensing error in addition to the current tailout effect is the necessity to provide a minimum conduction interval for the power switch to allow turn-off loss reduction networks normally included in converter circuits to properly reset. Normally with peak current control, the conduction interval becomes shorter and shorter during a current overload, and this reduction of the ONTIME is aggravated by the effect of the current sensing error, leaving insufficient time for turn-off loss reduction networks to properly reset.
One solution to the above-described problem discussed in U.S. Pat. No. 4,357,572, issued Nov. 2, 1982, to B. E. Anderson et al. and assigned to the same assignee as this application, involves continuously comparing an integrated value of an inductor voltage sensed during a nonconducting interval of a converter power switch, with a peak value of the filter inductor voltage sensed during a conducting interval of the power switch. When the integrated value attains the value of the peak value, an inhibit signal is removed from the clock supplying drive initiation pulses for the power switch. This control technique assures that the power switch in each cycle of operation during an overload has a substantially constant minimum ONTIME and a lengthened OFFTIME interval and, hence, restricts current tailout.